External cavity semiconductor lasers are known and have numerous uses and applications, including fiber-optic communications. In external cavity diode lasers which are typical of such optical devices, an optical cavity extends between a first facet of a semiconductor diode laser and an external reflector, defining the cavity ends. Another facet of the semiconductor laser, between the reflector and the first facet, typically carries an anti-reflection coating to allow light to escape the laser chip with minimum reflection.
Semiconductor diode lasers have been used extensively as transmitters for fiber-optic communications. In one common and low cost implementation, edges of two opposing end facets of the laser chip are cleaved to form resonant reflective surfaces and provide the feedback necessary for laser operation. Such Fabry-Perot (FP) lasers typically emit in multiple longitudinal modes and have large output bandwidths, for example, 3 nm to 10 nm. In another common implementation with slightly increased complexity, a Bragg grating is etched in the active region of the Fabry-Perot laser cavity to form a distributed feedback laser (DFB). Distributed feedback lasers have the advantage of single longitudinal mode emission which provides very narrow bandwidths typically, for example, less than 0.01 nm. In a third application, the distributed Bragg reflector (DBR) substitutes a wavelength-selective Bragg grating for one of the cleaved facets of the Fabry-Perot laser. The wavelength-selective Bragg grating has the effect of producing a laser with single longitudinal mode output.
Application of these and other diode lasers has been impeded due to inadequate stability and accuracy in the particular wavelengths generated. In particular, for example, such difficulties have been experienced in the application of diode lasers in Dense Wavelength Division Multiplexing (DWDM). In this advanced fiber-optic communication technology, many closely spaced wavelengths or channels are transmitted simultaneously down a single fiber or fiber bundle. Typical spacing of channels in DWDM systems can range from 5 nm to as little as 0.8 nm or less between channels, with closer channel spacing envisaged. To accomplish effective DWDM systems, stable and accurate transmitters of selectively predetermined wavelengths are needed for individual channels. In addition, stable and accurate wavelength-selective receivers are needed to selectively remove or receive the individual channel wavelengths with low or no cross talk from other channels. For a DWDM system to operate efficiently, therefore, the transmitter and receiver device for a given channel must be tuned with great accuracy to the same wavelength band.
Unfortunately, the wavelength band emitted by presently known semiconductor diode lasers, including the above mentioned FP lasers, DFB lasers and DBR lasers, vary to an unacceptably large degree with temperature and other factors. Center wavelength temperature dependence of an FP laser, for example, is typically as much as 0.4 nm per degree centigrade change in operating temperature. The comparable variance for DFB lasers is typically as much as 0.1 nm per degree centigrade. Presently known semiconductor diode lasers also suffer the disadvantage of poor manufacturing repeatability. That is, an intended or specified emission wavelength is not achieved with adequate accuracy when such lasers are produced in large commercial quantities. These deficiencies render present semiconductor diode lasers difficult and costly to implement into demanding applications such as DWDM systems, and in many cases entirely unsuitable.
It is known that the temperature dependence of an individual laser can be mitigated by controlling the temperature of the laser to within an extremely small temperature range using, for example, thermoelectric coolers with closed loop feedback from a temperature sensor. Such controls are complex and costly. The even more difficult problem of controlling lot-to-lot wavelength variation in commercial manufacturing of presently known semiconductor diode lasers, which can be as great as .+-.10 nm, has been partially addressed by culling through production batches for lasers having the desired wavelength. This technique of wavelength testing of individual lasers has significant adverse impact on manufacturing yield, with correspondingly increased costs and complexity.
It has also been proposed to use an alternative type of semiconductor diode laser, specifically, tunable external cavity lasers (ECL's). Tunable ECL's are suggested, for example, in Widely Tunable External Cavity Diode Lasers, Day et al, SPIE, Vol. 2378, P. 35-41. In the diode laser devices suggested by Day et al, an anti-reflective coating is placed on one facet of a diode laser chip. The emitted light is captured by a collimating lens, and a diffraction grating, acting in part as an external cavity reflector, is used to select or tune the wavelength of the laser. Laser action occurs, generally, provided that the grating is selecting a wavelength within the diode's spectral gain region. A diode laser device employing a diffraction grating disposed in an external cavity also is suggested in U.S. Pat. No. 5,172,390 to Mooradian. Unfortunately, diffraction gratings disposed within the external cavity of a diode laser create a significant increase in the overall size or bulk of the device. The diffraction grating and the complexity of the required grating alignment system can also significantly increase the cost of the device. As to the size or bulk of the device, the cavity length for a diode laser having a diffraction grating disposed in an external resonant cavity, in accordance with known devices, is typically from 25 mm to over 100 mm, in contrast to the much smaller 1 mm size or smaller of FP lasers and DFB lasers discussed above. The diffraction grating and grating mount also have been found to exhibit temperature dependence. Since the diffraction grating sets the wavelength of the laser, such temperature dependence of the grating and grating mount cause unwanted instability in the emitted wavelength of the laser. In addition, long term wavelength drift problems have been experienced due, it is believed, to the mechanical complexity of the diffraction grating and grating mount aspects of such devices.
Another known tunable ECL incorporates a Fabry-Perot thin film interference filter in the external cavity. The filter passband defines the resonant oscillation in the cavity and thus the operating wavelength of the ECL. As reported, e.g. by Zorabedian et al., Interference-filter-tuned, alignment-stabilized, semiconductor external-cavity laser, Optics Letters, Vol. 13, No. 10, pp 826-828 (10/88), wavelength tuning is accomplished by tilting the filter (see FIG. 11 herein). Such tilting, however, results in a change in the optical path distance through the filter assembly which does not correspond to the rate of change of the wavelength so that the tuned wavelength values jump by an amount corresponding to the adjacent mode spacing of the external cavity of the device. This effect is known as mode-hopping. Moreover, single mode filter-based ECL's (FECL's) are not reported, due to the unavailability of sufficiently narrow bandwidth filters and the limiting physical dimensions of ECL components like diode laser chips, filter/substrates, beam shaping optics and cavity reflectors. Thus the physical length of typical FECL's, which in inverse proportion relates to longitudinal cavity mode spacing, and limiting filter technology, have provided for bulky, multimode devices.
It is an object of the present invention to provide external cavity laser devices having good wavelength stability and accuracy, compact size (i.e., shorter cavity length and device volume), single mode output, and additionally, continuous, mode-hop-free tuning. In particular, it is an object to provide such devices having advantageous manufacturing costs and reduced complexity. Additional objects of the invention will be apparent from the following disclosure and from the detailed description of certain preferred embodiments.